Superconducting synchronous machines, which use a superconducting material for a field system or an armature, have attracted attention e.g. for the reason that a high power output can be obtained with high efficiency, and studies and proposals have been made thereon. As with common synchronous machines not using a superconducting material, superconducting synchronous machine can be classified broadly into a radial-gap type and an axial-gap type. Patent document 1 (Japanese Patent No. 5162654), for example, discloses a superconducting synchronous machine of the radial-gap type (hereinafter referred to as a radial-gap type superconducting synchronous machine), and patent document 2 (Japanese Patent Laid-Open Publication No. 2004-235625) by the present inventors, for example, discloses a superconducting synchronous machine of the axial-gap type (hereinafter referred to as an axial-gap type superconducting synchronous machine).
The radial-gap type superconducting synchronous machine of patent document 1 is a rotating field system-type synchronous machine in which a field system portion of a rotor is comprised of a permanent magnet, and an armature coil, provided in an armature as a stator, is comprised of a superconducting coil made of a superconducting material.
In the radial-gap type superconducting synchronous machine, a higher electric current can flow in the superconducting coil as compared to a copper coil, or the like, used in a common synchronous machine. Therefore, a large magnetic field can be generated by allowing the superconducting coil to function as an electromagnet. This makes it possible to produce a high power output with high efficiency.
On the other hand, the axial-gap type superconducting synchronous machine of patent document 2 has rotors and stators arranged alternately in the rotational axis direction of the rotors (a stator, a rotor and a stator are arranged in this order in an illustrated embodiment). The field system portion of the rotor is composed of a bulk superconductor.
A bulk superconductor, which is a mass of superconductor crystals, can capture magnetic flux lines at pinning points therein when a magnetic field (magnetic flux) is introduced into the bulk superconductor at a temperature which is not more than a critical temperature at which the matrix superconductor shows a superconducting transition. This enables the bulk superconductor to act as a magnet having a higher magnetic flux density than a permanent magnet. Thus, according to the synchronous machine, a field system portion having a high magnetic flux density can be obtained by allowing the bulk superconductor to capture magnetic flux lines, whereby a high power output can be obtained with high efficiency. Further, the bulk superconductor can hold a stronger magnetic field than a superconducting coil provided that the superconducting coil has the same size as the bulk superconductor. The synchronous machine therefore has an advantage in terms of downsizing over a synchronous machine which uses a superconducting coil. Furthermore, the synchronous machine does not require connecting wiring for supplying electric current to the field system portion. The synchronous machine therefore has advantages also in terms of simplification of the device structure and enhancement in the efficiency of the device system, such as reduction in the amount of incoming heat.
The axial-gap type superconducting synchronous machine of patent document 2 also has an advantage in that an armature coil, provided in an armature as a stator, is configured to function also as a magnetizing coil (magnetizing apparatus) for the bulk superconductor. Thus, the magnetizing apparatus is integrated with the synchronous machine. This makes it possible to magnetize the bulk superconductor conveniently in a timely manner.
The axial-gap type superconducting synchronous machine of patent document 2, in performing its magnetization with the armature coil, uses pulse magnetization in order to prevent the magnetizing apparatus from becoming large-sized upon the integration of the magnetizing apparatus with the synchronous machine and thereby losing the practical utility. A superconductor may be magnetized by pulse magnetization or static magnetic field magnetization. In the case of pulse magnetization, a strong magnetic field is instantaneously applied to a bulk superconductor, which is held at a temperature lower than its superconducting critical temperature, to introduce magnetic flux into the bulk superconductor. The bulk superconductor is allowed to capture the magnetic flux by the pinning effect, so that the bulk superconductor will function as a magnet having a high magnetic flux density. In the case of static magnetic field magnetization, a static magnetic field (stationary magnetic field) is applied to a bulk superconductor, which is held at a temperature higher than its superconducting critical temperature, to introduce magnetic flux into the bulk superconductor. The temperature of the bulk superconductor is then lowered to a temperature lower than the superconducting critical temperature, and the bulk superconductor is held at that temperature to allow the bulk superconductor to capture the magnetic flux by the pinning effect, thereby allowing the bulk superconductor to function as a magnet having a high magnetic flux density. In general, an object to be magnetized, such as a bulk superconductor, can capture more magnetic flux lines by static magnetic field magnetization than by pulse magnetization. However, in order to generate a necessary high static magnetic field in static magnetic field magnetization, it is necessary to produce a coil commensurate with the size of an object to be magnetized, and to cause a high electric current to flow in the coil. Further, magnetization of the object is performed by applying the static magnetic field to the object for a long time. The use of static magnetic field magnetization thus requires a large-scale magnetizing apparatus using a superconducting coil. For these reasons, the axial-gap type superconducting synchronous machine of patent document 2 uses pulse magnetization to magnetize the bulk superconductor with the armature coil. The practical utility can be ensured by the downsizing and integration of the magnetizing coil.